The invention relates generally to monitoring the conditions in a borehole of a subterranean well. Specifically, the invention is related to techniques for estimating material density within the annular space of a subterranean well.
Subterranean areas of interest beneath the surface are accessed through a borehole. The boreholes are surrounded by subterranean material, such as sand, that may migrate out of the boreholes with the oil, gas, water, and/or other fluids produced by the wells. A casing is inserted in a borehole and is held into place by cementing space between the outer surface of the casing and the surrounding earth. The borehole may also include other piping such as production tubing, and inner casing, and conductor casing inside the outermost casing. The fluid produced from the well flows to the surface through the production tubing. During the life of a subterranean well, the production tubing may have to be removed for repair and maintenance activities. There may also be a need to remove a portion or all of one or more of the other piping of the subterranean well.
The presence of sand and other particulate material may affect the functioning of various producing equipment, such as tubing, pumps, and valves. The particulate material may partially or fully clog the well thus reducing the fluid production capabilities of the wells. Maintenance of wells in such scenarios is expensive. The presence of the particulates in the hydrocarbon fluids from the wells necessitates additional processing at the surface thus increasing the cost of extraction of fluids.
Boreholes are suitably designed and constructed to prevent mixing of particulates with the fluids and are to be monitored for effectiveness of the design through the life of the well. Borehole design includes providing a perforated base pipe positioned proximate to the formation site of interest. A screen is disposed around the perforated base pipe and a coarse particulate material, such as sand, or proppants, which are typically sized and graded and collectively referred to as “gravel,” is disposed in the subterranean well between the screen and the borehole. The formation fluid flows through the screen and the gravel in the pack prevents formation fines and sand from flowing into the borehole and mixing with the produced fluids.
Over time, both distribution and density of the gravel in the borehole annulus can change for various reasons. For example, finer sand or other such particulate materials may enter and block the screen openings. The material of the gravel pack may be non-uniformly distributed due to borehole conditions such as non-uniform flow rates. During the formation of the gravel pack or during the operation of the well over an extended period of time, void areas may be created in the material around the borehole. Non uniform material distribution around the borehole would increase the possibility of introduction of particulate materials in the extracted fluid.
During the production of the fluid, drilling fluids fill the annular space between the concentric pipes. Particulates within the drilling fluids may precipitate within the annular space between two successive concentric pipes. Over a longer period of time, such particulates along with fluid and oil, may form a cement like substance that couples together the concentric pipes. Such coupling prevents removal of the inner pipes from the wellbore. In some situations, cutting tools are used to sever the pipes to enable removal of the production tubular, inner casing or other pipes. The pipes are to be cut at a depth above where the coupling is preventing the removal of the pipes.
The material of the borehole annulus is to be monitored continuously for effective prevention of mixing of the particulate materials with the fluid. Accurate estimation of material density in the annulus would help to foresee problems in the borehole and take effective steps at an optimal cost. There is a need to devise techniques for estimating the density of the material in the annulus in real time.